This invention relates to apparatus and technique for measuring the growth of turbine blades of a gas turbine engine.
Because of the hostile environment to which the turbine blade of a jet engine is subjected, it has over the years been an almost impossible task to achieve its growth measurement with any degree of accuracy. As is well known, the growth is a vital parameter as it is an indication of its life and more importantly its life expectancy. Should a blade exceed a predetermined growth, it becomes vulnerable to destruction and for maintenance and safety reasons it is desirable to replace such blade before such occurrence. Obviously, if the measurement is not accurate the blade may either be prematurely discarded or that it could be re-installed having a life expectancy level beyond the limit of the part resulting in premature blade fracture. It has been desirable to achieve absolute accuracy and repeatability in this measurement.
While it would be desirable to make a direct physical measurement of the blade growth by conventional measuring instruments such as a micrometer, the blade structure in many instances precludes such measurements. Other techniques, like scribing a line on the face of the blade is also not acceptable inasmuch as it would not only introduce a stress area but as a result of erosion caused by the extreme operating environment, the scribe line would become obscured in time. Placing a deeper groove to counter this problem would only make matters worse. A deep groove is very difficult to coat with corrosive and oxidation preventative materials, thereby allowing for early corrosive attack and subsequent failure.
We have found that we can achieve a high degree of accuracy and repeatability in blade growth measurement and obviate the above problems by judiciously locating a dimple on the concave outer surface of the blade. The dimple is so conformed that the erosion and wear of the blade does not affect its availability after repeated usage. The absolute surface condition of the blade in no way influences the measurement to be taken. The location of the dimple makes the growth measurement independent of the actual physical condition of the blade. Moreover, the dimple does not in any way affect the stress or aerodynamics of the blade.